As is well known in the art, optical fibers include glass base optical fibers using quartz glass and multi-component glass as core and clad components and plastic optical fibers using plastics as core and clad components. As compared with the former, the plastic optical fibers have some drawbacks including an increased transmission loss undesirable for long distance communications, but some advantages including an increase of numerical aperture, ease of handling, and low cost.
Conventional plastic optical fibers generally include cores formed of organic polymers having high light transmittance and clads formed of highly transparent organic polymers having a lower index of refraction than the core polymers. The organic polymers having high light transmittance suitable as the core are typically poly(methyl methacrylate) and polystyrene. It was recently proposed to use organopolysiloxane elastomers as the core material as disclosed in Japanese Patent Application Kokai No. 43613/1985. Plastic optical fibers using such elastomers as the core have advantages over those using poly(methyl methacrylate) and other conventional core materials because of higher flexibility and higher thermal resistance.
Nevertheless, the organopolysiloxane elastomers applied as the core of plastic optical fibers have a drawback in that light transmission loss is increased within a short period of time when exposed to high humidity conditions. If external pressure is applied to the optical fibers so as to compress them, then the light transmission loss is increased in proportion to the external stress. It is thus desired to eliminate these drawbacks.